Stents are frequently used in the medical field to open vessels affected by conditions such as stenosis, thrombosis, restenosis, vulnerable plaque, and formation of intimal flaps or torn arterial linings caused by percutaneous transluminal coronary angioplasty (PCTA). Stents are used not only as a mechanical intervention, but also as vehicles for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand a vessel wall. Stents may be capable of being compressed in diameter, so that they can be moved through small vessels with the use of a catheter or balloon-catheter, and then expanded to a larger diameter once they are at the target location. Examples of such stents include those described in U.S. Pat. No. 4,733,665 to Palmaz, U.S. Pat. No. 4,800,882 to Gianturco, U.S. Pat. No. 4,886,062 to Wiktor, U.S. Pat. No. 5,514,154 to Lau et al., and U.S. Pat. No. 5,569,295 to Lam.
A stent must have sufficient radial strength to withstand structural loads, such as radial compressive forces, imposed on the stent as it supports the walls of a vessel or other anatomical lumen. In addition, the stent must possess sufficient flexibility to allow for crimping, deployment, and cyclic loading from surrounding tissue. Also, a sufficiently low profile, that includes diameter and size of struts, is important. As the profile of a stent decreases, the easier is its delivery through an anatomical lumen, and the smaller the disruption in the flow of blood or other bodily fluid.
Metal stents typically stay implanted in a patent for a longer amount of time than bioresorbable polymer stents. For example, metal stents can be implanted for years or permanently in a patient. Since bioresorbable polymer stents degrade over time, they gradually allow for native positive remodeling of the anatomical lumen, which involves allowing the anatomical lumen to enlarge naturally. On the other hand, metal stents can prevent positive remodeling.
Stents made of bioresorbable polymers have been developed to allow for improved healing of the anatomical lumen. Examples of bioresorbable polymer stents include those described in U.S. Pat. No. 8,002,817 to Limon, U.S. Pat. No. 8,303,644 to Lord, and U.S. Pat. No. 8,388,673 to Yang.
Scaffold designs for stents made of bioresorbable polymers involve a balance between radial strength and expansion capability. Bars that are shorter in length generally provide greater radial strength, in that the scaffold can withstand a greater inward radial force without collapsing to a smaller diameter from a fully deployed state, as compared to a scaffold having bars that are longer in length. However, the increase in radial strength provided by shorter bars usually comes at the expense of expansion capability. A scaffold with longer bars provides increased expansion capability, in that the scaffold provides less resistance to being expanded from a fully crimped state to a fully deployed state, as compared to a scaffold with shorter bars. The increase in expansion capability provided by longer bars usually comes at the expense of radial strength.
Accordingly, there is a continuing need for stent strut configurations and manufacturing methods that facilitate delivery of polymer stents with maximized radial strength and expansion capability.